1. Field of the Invention
The present invention relates generally to power assisted steering systems such as are used in automotive vehicles, and more particularly to a method and apparatus for suppressing vibrations in the steering system that may otherwise be transmitted to the steering wheel.
2. Background
Automotive vehicle steering systems are subject to a large number of forces that may cause vibrations that are felt by the driver through the steering wheel. These undesirable steering wheel vibrations, commonly known as nibble, may be caused by brake roughness, tire/wheel imbalance, road input, or other excitations.
Since steering nibble is caused by excitations over a wide range of different frequencies, an effective engineering solution to the problem is very challenging. Previous efforts to eliminate nibble have focused on designing and tuning the elastomeric bushings, bearings, and joints used throughout the steering and suspension systems to achieve a desired level of vibration damping. Unfortunately, these efforts have had only limited success, and they often require compromises in other vehicle attributes such as ride, steering feel, rolling comfort, and impact harshness.
Oil-filled elastomeric bushings, also known as hydraulic bushings, are the primary type of bearing used to reduce suspension vibration caused by brake and tire/wheel systems. However, hydraulic bushings require a relatively large amount of packaging space, so that major structural and chassis system redesigns may be required to accommodate these components. Also, hydraulic bushings are expensive and not very durable.
U.S. Pat. Nos. 6,120,046 and 6,561,302 teach mechanisms that apply a torsional damping force to a steering shaft or column that connects a steering wheel to a steering assembly (such as a rack and pinion assembly). This torsional damping directly inhibits rotation of the steering wheel and therefore may have an undesirable effect on steering feel. Also, torsional damping of the steering wheel/column assembly is unable to effectively damp vibrations if the amplitude of such vibrations is below the threshold of steering angular displacement necessary to activate a power assist system. This is most likely to be the case with higher frequency vibrations.
U.S. Pat. No. 6,491,313 teaches a variable hydraulic damper intended for attachment to a rack or center link of a steering system. The amount of damping provided by the device is varied by controlling the size of an orifice through which fluid flows between the opposite sides of a piston. The size of the orifice is varied in inverse proportion to the amount of hydraulic pressure being supplied to the power assisted steering system. This results in greater damping being applied to the steering system when a relatively low degree of power assist is called for (such as at higher vehicle speed and/or neutral steering wheel angle positions), and less damping being applied when a higher degree of power assist is called for (such as at lower vehicle speed and/or greater steering wheel angle displacement). This device overcomes the limitations inherent in torsional dampers pointed out in the paragraph above, but a hydraulic damper has its own inherent limitations in regards to the specific types of vibrations that it can effectively damp.
The present invention provides an effective means for reducing steering nibble and other vibrations in a power-assisted steering system without changing or compromising any other performance attributes of a vehicle""s steering or suspension systems. In general, this is achieved by providing a damping member in close proximity to a steering actuator, and a damping control system operative to vary the force with which the damping member is urged against the steering actuator. The damping control system increases the amount of damping force when the power steering boost system is applying little or no steering force to the steering actuator, and reduces the amount of damping force when the power boost system is applying significant steering force to the steering actuator.
In this way, the steering actuator is damped to significantly reduce steering nibble whenever the driver of the vehicle is not providing any steering input to the steering system, as when the vehicle is tracking straight ahead or is in a constant radius turn. However, when the driver is providing steering input, as when turning the steering wheel, the steering actuator is substantially undamped so as not to interfere with the xe2x80x9cdriving feelxe2x80x9d sensed by the driver through the steering wheel.
In a preferred embodiment of the invention disclosed herein, the steering actuator is enclosed by a housing and is moved relative to the housing along an axis by a hydraulic boost system in order to apply steering force. The damping member is formed of a polymer material, surrounds the steering actuator, and has a frustroconical outer surface. At least one clamping member is disposed between the damping member and the housing and is movable parallel to the steering actuator""s axis of movement between a damping position wherein the clamping member is wedged between the inner surface of the housing and the outer surface of the damping member to urge the damping member against the steering actuator, and a released position wherein the clamping member does not urge the damping member against the steering actuator.
A spring biasing mechanism, which may be a coil spring, urges the clamping element toward the damping position, and a shuttle valve operates to regulate pressure from the hydraulic steering boost system, increasing the pressure in a chamber within the housing to overcome the spring force whenever hydraulic pressure is applied to the steering actuator.
According to another feature of the invention, the spring biasing mechanism is operative to allow the spring preload to be varied. This is preferably accomplished by an electronic control unit (ECU) that receives inputs from a driver-activated switch to allow the driver to select the amount of damping. The ECU may also receive inputs from one or more vehicle condition sensors, such as a vehicle speed sensor or a yaw rate/acceleration sensor, to allow automatic, adaptive adjustment of the degree of damping in accordance with a control algorithm designed to optimize driving comfort and safety.
Other features and advantages of the present invention will be readily appreciated and better understood after reading the subsequent description when considered in connection with the accompanying drawings.